1. Field of the Invention
This invention relates to a radar fill-level sensing device encompassing a high-frequency unit that serves to generate and process radar signals at a predefined frequency and wavelength, an antenna for sending and/or receiving radar signals, as well as board-mounted microstrip circuitry for connecting the high-frequency device to the antenna, said microstrip circuitry including a ground conductor. A radar fill-level sensing device of this type has been described earlier, for instance in U.S. 2004/0066588 A1.
2. The Prior Art
Industrial applications often involve the need to measure the fill level of a substance, for instance a liquid or a bulk material, in a container such as a tank. This has been accomplished by a variety of techniques, differentiated between contact and non-contact level sensing methods. Contact level sensing methods whereby a component of the sensing device makes contact with the substance whose level is to be determined, include level sensing for instance, by means, of a float or a feeler plate. Contact level sensing also includes capacitive measurements in which the level-dependent capacity between an electrode immersed in the substance and the container wall is measured, as well as thermal measurements in which the increased heat dissipation upon immersion of a current-carrying, temperature-dependent resistance element into the substance serves as an indicator based on the fact that the electrical resistance changes as a function of the depth of immersion.
Non-contact fill-level sensing techniques include measurements, for instance, using a laser or ultrasound. In that case, a laser or ultrasound signal is transmitted, it reflects back off the surface level of the substance and the reflected signal is recaptured, its run time providing an indication of the fill level of the substance. Based on that same principle, the radar fill-level sensing method involves the generation of a microwave signal that is transmitted, via an antenna such as a rod or horn antenna, toward the substance whose fill level is to be gauged and whose surface reflects the signal, which is then recaptured by that same antenna or by another antenna.
There are in fact different radar fill-level measuring techniques. In the case of pulsed-radar level sensing, a microwave signal is transmitted in the form of short pulses which are either unmodulated or carrier-frequency modulated. The runtime of the microwave pulses from the transmitter to the surface of the substance and back to the receiver indicates the distance between the transceiver and the substance. The transceiver typically employs a single antenna. In the case of frequency modulated continuous-wave (FMCW) radar level sensing, the microwave signal is emitted in continuous fashion but its frequency is modulated, typically with consecutive ramping. Due to the time delay during signal propagation, the transmitted frequency will have changed by the time the reflected signal is received, with the differential frequency being an indicator of the distance from the reflecting surface and thus of the fill level. Then there is time domain re-flectometry (TDR) radar level sensing which is similar to the pulsed radar level sensing technique except that it is usually line-bound, employing electrical pulses without a carrier frequency.
Before the radar signal can be transmitted by the antenna in the direction of the surface of the substance whose level is to be determined, that radar signal must be transmitted from the high frequency generator to the antenna. That requires hard-wired conductors such as a coaxial cable that is composed of an inner conductor and an outer conductor with a dielectric in between, or a parallel line in which two conductors are held parallel to each other either by spacers or by a dielectric jacket. Also known are hollow waveguides consisting of a metal tube with a circular or square cross section allowing the electromagnetic high-frequency radar signal to propagate. The interior of the hollow waveguide may be filled with air or with a dielectric.
Finally, there have been flat conductors in the form of planar circuitry printed on a dielectric substrate. Such planar conductors may be in the form, for instance, of board-mounted microstrip conductors, in which case one side of the board supports the actual waveguide structures while the other side of the board is provided with a metallic coat that is separated from the said waveguide structures by the dielectric board. The advantage of planar circuits such as microstrip conductors is that they also allow for the simple planar mounting of other components on the board.
It is especially in the chemical and related industries that the radar level sensing device, along with other measuring devices, must be explosion-proof. For radar level sensing devices such explosion protection is particularly necessary in cases where, for example, the fill-level of an explosible substance contained in a tank is to be measured. DIN EN 50014 specifies standards for various ignition-protection classifications that govern electrical equipment in potentially explosive areas.
One of these ignition protection classifications is referred to as “intrinsically safe” or, for short, “ex-i”. In the “intrinsically safe” protection category, the energy in intrinsically safe circuits is limited to a point where, neither in normal operation nor in the event of a failure such as a short-circuit, the conductors would be capable of igniting explosive mixtures, for instance, due to inadmissibly high temperatures, sparking or arcing. The intrinsic safety requirement is not limited to an individual component, but pertains to the intrinsically safe design of the complete circuitry system.
Circuitry of a corresponding, intrinsically safe design includes, apart from the interconnecting conductors, at least one intrinsically safe power supply and an associated accessory unit. The term intrinsically safe power supply refers to a power supply with only intrinsically safe circuit elements. What this means is that in the intrinsically safe power supply the voltage and current will always be so small that in the event of a is short circuit, a power interruption or a ground fault, the ignition energy will always remain below the minimum ignition energy, rendering the ignition of an explosible gas mixture impossible.
Intrinsically safe power supplies can be used for operation directly in a potentially explosive area. By contrast, the term associated accessory unit refers to modules in which not all circuit elements are intrinsically safe. However, an associated accessory unit encompasses at least one intrinsically safe circuit that may extend into the potentially explosive area. Accordingly, a signal that is not intrinsically safe is converted into an intrinsically safe signal. Examples of associated accessory units include pure signal isolators, commonly referred to as safety barriers, or signal-converting devices such as measuring transducers, transmitter feed units or isolation transformers.
In the case of the radar level sensing device described in the abovementioned U.S. 2004/0066588 A1, the board-mounted microstrip circuitry that serves to connect the high frequency unit to the antenna can be made intrinsically safe by means of a ground conductor that provides effective protection for low-frequency signals and direct-current signals. The part of the microstrip circuitry that serves as the ground conductor, having a length of essentially one quarter of the predefined wavelength of the radar signals, branches off at one end from the actual connecting line between the high frequency unit and the antenna, respectively, while its other end is grounded by through-contacts through the board connecting to a grounding layer on the other side of the board. Appropriate dimensioning of this ground conductor especially in terms of adequate width and safe grounding can render the circuitry intrinsically safe since it functions as a grounding element for low-frequency signal components or DC components, whereas the high-frequency signal components, not “recognizing” that type of grounding, remain unaffected.
However, the drawback of the design described in U.S. 2004/0066588 A1 is its poor reproducibility. Moreover, grounding provisions of the type described are difficult to simulate since drilling through the board and, respectively, providing the other side with the grounding layer must take into account the numerical simulation methods employed, which significantly adds to the complexity of the process.